The lung is the site of infection by influenza (flu) virus which gains access to airways by aerosol transmission. This infection is confined to lung epithelium, and its control depends upon an effective, local immune response that is mediated by CD4 and CD8 cells. Protective immunity is associated with flu-specific CD4 and CD8 cells that persist for extended durations in airways. Thus, optimal vaccine development to organisms such as flu that could be used for bioterrorism will depend upon greater understanding of the requirements for generating effector cells with the capacity to migrate into, as well as respond and persist in the target tissue. CD44, the product of a single gene that is subject to mRNA splicing and post translational modifications, is a key regulator of inflammatory processes in the lung. Expression of high levels of CD44 is the most widely used marker to distinguish effector and memory T cells from naive T cells, yet little is known regarding it's function in Ag-experienced cells. Our preliminary studies of responses to flu using CD44- deficient mice and CD44-blocking antibodies support the hypothesis that CD44 is essential for T cell migration into lung airways, and provide evidence for additional roles in regulating the responses and survival of effector and memory cells. We have developed adoptive transfer models with TCR transgenic CD4 and CD8 cells with which we can induce and track Ag-specific T cell responses to engineered flu viruses that express the appropriate peptides. With Class I and Class 1I multimers and cytokine reporter mice we can also follow responses ofpolyclonal T cells. With these tools, we propose to investigate mechanisms by which CD44 can contribute to T cell immunity to flu. Aim 1 will test the hypothesis that CD44 mediates an essential adhesion mechanism that enables the transmigration of effector T cells into the airways in response to infection and functions in the maintenance of flu-specific T cells in this site. Aim 2 will test the hypothesis that CD44 regulates cytokine production by effectors, and supports their appropriate expansion and responses to flu within the lung. Aim 3 will investigate the role of CD44 in regulation of the homeostasis oft cell subsets. We believe that these studies will provide new insights into mechanisms by which adhesion pathways and interactions with extracellular matrix components can affect the optimal development of T cell responses in target tissues as well as the longevity of memory.